1. Field of the Invention
This invention relates to the treatment of liquid and gaseous wastes, and more particularly to the treatment of hydrocarbon waste materials in a fluid through a corona discharge, and the generation of a corona discharge with a radio frequency (RF) operated helical coil quarter-wave resonator.
2. Description of the Related Art
Corona scrubbers have been used to activate chemical reactions that break down pollutants in a gas discharge. Such systems, which are described for example in Nunez et al. "Corona Destruction: an Innovative Control Technology for VOCs and Air Toxics", Air & Waste, Vol. 43, February 1993, pages 242-247 and Yamamoto et al., "Control of Volatile Organic Compounds by an AC Energized Ferroelectric Pellet Reactor and a Pulsed Corona Reactor", Industry Applications Society Annual Meeting 1989, Vol. 2, pages 2175-2179, employ relatively low frequency energizing signals to produce a corona discharge. Energizing frequencies that have been used generally range from the standard line frequency of 60 hz up to about 200 Hz. Unfortunately, this corona discharge technique has not been found to be applicable for the treatment of waste products contained in a liquid.
In the field of liquid waste treatment, chlorination is commonly used for potable water and sewage. However, it has serious disadvantages in terms of safety, handling complexity, and the generation of undesirable chlorinated hydrocarbons as byproducts; other chemical treatments have similar drawbacks. Another approach uses ultraviolet (UV) excitation to destroy biologically active viral and bacterial agents remnant in sewage. However, UV has not been shown to be of utility in the decontamination of industrial or sewage waste requiring the removal of destruction of carcinogenic or toxic compounds. Thermal treatments, such as distillation, have also been investigated but are very expensive.
Another area of investigation is the treatment of contaminated wastewater with electron beams. This type of treatment has been demonstrated using a 1.5-MeV electron beam scanned across a thin sheet of flowing water, as described in W. J. Cooper, et al. "Treatment of Industrial Hazardous Wastes With High Energy Electrons", presented to Hazardous Materials Control Research Institute's 7th National RCRA/Superfund Conference, May 2-3, 1990, St. Louis, Mo., pages 1-15. The technique has been shown to be effective against chlorinated hydrocarbons and many other organic contaminates, which are reduced or oxidized to inert compounds by the action of free radicals and free electrons induced as secondaries within the water by the beam. High energy electrons deposit energy into the water by bremsstrahlung radiation, which creates low energy ionizing x-rays, and by ionizing collisions. The exact chemical processes are complex, but they are believed to lead to the formation of a variety of reactive species within the water, in particular to free thermal electrons and to OH radicals, that are highly reactive.
Electron beam treatment has required a high beam energy to obtain a suitable penetration depth. This in turn involves a high cost to shield x-rays and to erect the structure. At lower electron beam energies, in the 100-150 keV range, the power supply and electron gun become of more manageable size and the x-ray hazard becomes manageable. However, beam losses in the window that protects the electron gun vacuum become serious below 150 kV, with the robustness of the window vanishing below about 100 kV if it is designed to be thin enough to efficiently transmit electrons. In addition to the window injury, the low beam energy results in a penetration depth that is very short and requires exotic capillary fluid flow apparatus (with high viscous flow losses) to ensure that the fluid cross section is thin enough to be successfully irradiated. Furthermore, there is a serious foil-heating problem that arises with high duty use.
In addition to the gas scrubbing application mentioned above, corona discharge devices have been developed which have other potential environmental uses. For example, conventional spark plugs used in internal combustion engines typically deliver energies of about 20-30 mJ per pulse. If a higher energy ignition system could be developed, it would offer several advantages. First, with current systems not all of the burned gas is ejected from the cylinder during idle, resulting in a rough idle; increased idle stability could be achieved with a higher energy ignition. Second, higher fuel economies are available through exhaust gas recirculation (EGR) systems. With a spark energy of about 75 mJ, the EGR can be increased to its optimum level, and the gas mileage improved on the order of 1 MPG. In addition, NOx emissions would be reduced. Third, a higher energy spark would allow the fuel mixture at startup to be run leaner. Since most hydrocarbon emissions occur at startup, a significant drop in hydrocarbon emissions could be expected. Unburned fuel in the exhaust manifold, particularly when the catalytic converter is cold, is also an environmental problem that can be addressed by an efficient corona discharge.
One corona discharge device that is of interest is described in Bonnazza et al., "RF Plasma Ignitions System Concept for Lean Burn Internal Combustion Engines", Society of Automotive Engineers, Paper No. 929416, 1992, pages 4.315-4.319. It uses a co-axial, quarter-wave resonator with a solid inner conductor surrounded by an outer conductive cylinder. The outer cylinder is grounded, while a high frequency RF signal, in the hundreds of megahertz range, is applied to the inner conductor. The apparatus extends for one-quarter the length of the excitation wavelength, resulting in a co-axial cavity resonator with a maximum voltage at the opposite end of the device from where the RF input is applied. This produces a step-up transformation of the voltage at the opposite end of the device when resonance occurs.
The article reports the testing of a model, approximately 38 cm long, with a 200 MHz input signal that was applied through a rectangular loop feed. While successful ignitions were observed, the 38 cm length of the device was much longer than what is practical for a vehicle ignition system.